DESCRIPTION The Vibrio fischeri-Euprymna scolopes light organ symbiosis exemplifies typical animal bacterial interactions-stable, non-pathogenic bacterial colonization of host tissue with both the animal and its microflora able to recognize, signal, and developmentally influence one another. Several developmental changes in light organ tissue occur only in animals infected with V. fischeri, however the mechanisms underpinning bacterial colonization and induction of development are unknown. The aim of this proposal is to investigate the role of two noel ADP-ribosyltransferases (ADPrs) exported by V. fischeri, HvnA and HvnB, in this bacterium's ability to colonize and trigger developmental changes in E. scolopes. ADPRs covalently modify proteins, and endogenous ones regulate important cellular processes while known excreted bacterial ADPrs exert toxic effects on hosts in pathogenic interactions. We therefore hypothesize that ADPrs play a role in this symbiosis We propose to: (i) clone and characterize the hvnA and hvnB operons from a squid symbion, (ii) determine whether hvnA and hvnB genes are expressed in the E. scolopes light organ, (iii) test the ability of mutants lacking hvnA and/or hvnB to colonize and trigger development in E. scolopes, (iv) localize HvnA and HvnB within the light organ immunologically and (v) determine whether specific host proteins are ADP-ribosylated by HvnA and HvnB.